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//Saved me alot of stress this one. TLDR check bindValue
It depends on the type of schoolsArray. In your case, I guess, schoolsArray is a list of objects, while ngModel is an array of strings. ng-select doesn't know how to map strings from ngModel to objects from items.
Please see documentation for property bindValue to inform ng-select how to bind items keys to the model. https://github.com/ng-select/ng-select
To summarize, add bindValue="id-property-name-in-items" to ng-select tag.